Multi-focus optical-resolution photoacoustic microscopy with ultrasonic array detection

ABSTRACT

A probe for use with an imaging system, including a scanning device configured to receive a first light beam from a light source, a beam-divider configured to split the first light beam into a plurality of second light beams, and a focusing device configured to focus each of the second light beams on respective locations in an object of interest is disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of prior U.S. application Ser. No. 13/369,558, filed on Feb. 9, 2012, the entirety of which is incorporated herein in its entirety. U.S. application Ser. No. 13/369,558 claims the benefit of U.S. Provisional Application No. 61/442,148 filed Feb. 11, 2011, which is also incorporated herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

This invention was made with government support under grants R01 EB000712 and U54 CA136398, both awarded by the U.S. National Institutes of Health. The government has certain rights in the invention.

BACKGROUND

The ability to image microstructures, such as the micro-vascular network in the skin, the GI tract, or the brain cortex, and to monitor physiological functions of tissue is invaluable. One of the promising technologies for accomplishing this objective is photoacoustic microscopy. Current high-resolution optical imaging techniques, such as optical coherence tomography, can image up to approximately one transport mean free path (about 1 mm) into biological tissue. However, these techniques are insensitive to optical absorption that is related to important biochemical information. Other well-known techniques, such as confocal microscopy and multi-photon microscopy, often involve the introduction of exogenous dyes, which, with a few notable exceptions, have relatively high toxicity. In addition, acoustic microscopic imaging and spectroscopy systems are sensitive to acoustic impedance variations only, which have low contrast for early-stage cancer and provide little functional information about biological tissue except flow. In contrast, photoacoustic wave magnitude is, within certain bounds, linearly proportional to the optical absorption contrast; thus photoacoustic spectral measurement can be performed to gain functional (physiological) information such as the local blood oxygenation level.

BRIEF DESCRIPTION

In one aspect, an imaging method is provided the includes receiving a first light beam from a light source; splitting the first light beam into a plurality of second light beams using a beam-divider; focusing the plurality of second light beams on respective locations in an object of interest using a focusing device; and receiving the acoustic signals from the object of interest using an ultrasonic transducer array. The plurality of second light beams may cause the object of interest to emit acoustic signals. The plurality of second light beams and acoustic signals may be coaxially aligned on opposite sides of the object of interest in a transmission mode. The method may further include coaxially merging the plurality of second light beams and the acoustic signals using an optical-acoustic beam combiner. Coaxially merging the plurality of second light beams and the acoustic signals may include passing the plurality of second light beams through the optical-acoustic beam combiner to the focusing device and reflecting the acoustic signals entering the optical-acoustic beam combiner toward the ultrasonic transducer array. Coaxially merging the plurality of second light beams and the acoustic signals may include reflecting the plurality of second light beams entering the optical-acoustic beam combiner toward the focusing device and passing the acoustic signals through the optical-acoustic beam combiner to the ultrasonic transducer array. The method may further include reflecting the first light beam toward the beam-divider using a movable scanning mirror. The first light beam may be reflected in a raster scanning pattern. The method may further include generating an image based on the acoustic signals. The plurality of second light beams may be formed as a linear array. The acoustic signals may be received by a linear ultrasonic transducer array. The plurality of second light beams may be formed as a 2D array. The acoustic signals may be received by a 2D ultrasonic transducer array.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments described herein may be better understood by referring to the following description in conjunction with the accompanying drawings.

FIG. 1 is a diagram of a photoacoustic probe of an imaging system in accordance with one embodiment of the present disclosure, where a 2D transmission grating and an optically transparent acoustic reflector are employed.

FIG. 2 is a block diagram showing the overarching architecture of the present disclosure.

FIG. 3 is a diagram of the integrated focusing assembly of an imaging system in accordance with an alternative embodiment of the present disclosure, where a 2D transmission grating and a custom-built optical-acoustic beam combiner are employed.

FIG. 4 is a diagram of the integrated focusing assembly of an imaging system in accordance with another alternative embodiment of the present disclosure, where a 2D transmission grating and an acoustically transparent thin-film optical reflector are employed.

FIG. 5 is a diagram of the integrated focusing assembly of an imaging system in accordance with another alternative embodiment of the present disclosure, where a 2D transmission grating and a transmission-mode design are employed.

FIG. 6 is a diagram of the photoacoustic probe of an imaging system in accordance with another alternative embodiment of the present disclosure, where a 2D microlens array and an optically transparent acoustic reflector are employed.

FIG. 7 is a diagram of the integrated focusing assembly of an imaging system in accordance with yet another embodiment of the present disclosure, where a 2D microlens array and a custom-built optical-acoustic beam combiner are employed.

FIG. 8 is a diagram of the integrated focusing assembly of an imaging system in accordance with yet another embodiment of the present disclosure, where a 2D microlens array and an acoustically transparent thin-film optical reflector are employed.

FIG. 9 is a diagram of the integrated focusing assembly of an imaging system in accordance with yet another embodiment of the present disclosure, where a 2D microlens array and a transmission-mode design are employed.

FIG. 10A shows a photoacoustic image of two crossed 6-micrometer diameter carbon fibers acquired with the current prototype of the present disclosure.

FIG. 10B shows a distribution of the photoacoustic amplitude across the vertical fiber shown in FIG. 10A.

FIGS. 11A and 11B show in vivo photoacoustic images of a mouse ear vasculature acquired with the current prototype of the present disclosure.

FIG. 12 is a flowchart illustrating an exemplary imaging method.

DETAILED DESCRIPTION

While the making and using of various embodiments of the present disclosure are discussed in detail below, it should be appreciated that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the disclosure and do not delimit the scope of the disclosure.

To facilitate the understanding of this disclosure, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present disclosure. Terms such as “a,” “an” and “the” are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the disclosure, but their usage does not delimit the disclosure, except as outlined in the claims.

To be consistent with the commonly used terminology, whenever possible, the terms used herein will follow the definitions recommended by the Optical Society of America (OCIS codes).

In some embodiments, the term “photoacoustic microscopy” refers to a photoacoustic imaging technology that detects pressure waves generated by light absorption in the volume of a material (such as biological tissue) and propagated to the surface of the material. In other words, photoacoustic microscopy is a method for obtaining images of the optical contrast of a material by detecting acoustic or pressure waves traveling from the object. As used herein, the term “photoacoustic microscopy” includes detection of the pressure waves that are still within the object.

In some embodiments, the terms “reflection mode” and “transmission mode” refer to a laser photoacoustic microscopy system that employs the detection of acoustic or pressure waves transmitted from the volume of their generation to the optically irradiated surface and a surface that is opposite to, or substantially different from, the irradiated surface, respectively.

In some embodiments, the term “multi-focus 1D array illumination” refers to optical illumination for photoacoustic excitation with a one-dimensional array of focused pulsed laser beams.

In some embodiments, the term “multi-focus matrix illumination” refers to optical illumination for photoacoustic excitation using a two-dimensional array (matrix) of focused pulsed laser beams.

In some embodiments, the term “linear ultrasonic array” refers to a one-dimensional array of ultrasonic transducers, with which the two-dimensional (2D) in-plane spatial distributions and strength of ultrasonic (photoacoustic) sources can be reconstructed based on the time-resolved signals arriving at the array.

In some embodiments, the term “matrix ultrasonic array” refers to a two-dimensional array of ultrasonic transducers, with which the 3D spatial distributions and strength of photoacoustic sources can be reconstructed based on the time-resolved signals arriving at the array. Ultrasonic transducers generally refer to all types of ultrasonic wave detection devices including devices utilizing optical interferometers to sense ultrasonic waves.

In some embodiments, the term “diffraction limited resolution” refers to the best possible resolution by focusing light within the limitations imposed by diffraction.

In some embodiments, the term “photoacoustic emissions” refers to the pressure waves produced by light absorption.

In some embodiments, the term “B-scan image” refers to a cross-sectional two-dimensional image in the plane containing the acoustic axis.

In some embodiments, the term “integrated focusing assembly” refers to an integrated assembly including optical focusing components, an ultrasonic array, and the coupling devices between them.

In some embodiments, the term “photoacoustic reconstruction” refers to a signal processing technique used to reconstruct a photoacoustic B-scan image from received signals.

Embodiments of the present disclosure provide methods, systems, and apparatus for high-speed, optical-resolution photoacoustic imaging using multi-focus optical illumination in conjunction with ultrasonic array detection. Specifically, embodiments of the present disclosure use multiple focused pulsed laser beams to produce a rapid local temperature rise at multiple optical foci from absorption of the pulsed light. The temperature rise leads to a transient thermal expansion, resulting in photoacoustic emissions, which are detected by a high-frequency ultrasonic array to reconstruct an image. The image signal amplitude is related to the optical absorption and Grueneisen parameter. With each laser pulse, the multi-focus illumination excites photoacoustic waves from multiple sites, which the ultrasonic array detects simultaneously. The use of the ultrasonic array with photoacoustic reconstruction allows the separation of signals from the multiple optical illumination sites as close as the lateral resolution of the ultrasonic array. Fundamentally different from simply combining multiple assemblies of a single optical focusing element and a single ultrasonic transducer, this approach has enabled us to position optical foci much closer to each other.

Compared with previously existing optical-resolution photoacoustic microscopy, embodiments of the present disclosure significantly reduce the range or area of scanning for 2D or 3D imaging, respectively, by up to two to three orders, depending on the number of illumination spots. As a result, rapid scanning can be used to produce 2D or 3D photoacoustic imaging with optical resolution at high speed—even in real time. For example, using multi-focus matrix illumination in conjunction with matrix ultrasonic array detection, a 3D photoacoustic image can be produced by rapidly scanning the illumination over a small area comparable in size with the lateral resolution of the ultrasonic array. In fact, even with a simplified design using 1D array illumination in conjunction with linear ultrasonic array detection, the multi-focus optical-resolution photoacoustic microscopy device demonstrates a significant improvement in imaging speed over previously existing optical-resolution photoacoustic microscopy devices.

At an ultrasonic frequency suitable for ˜1 millimeter (mm) penetration in soft tissue, the optical diffraction limited spatial resolution can be two orders of magnitude finer than the acoustically defined counterpart. In addition, in comparison with a conventional broad light illumination, the confined illumination significantly reduces the background arising from the interference of photoacoustic waves from various targets within the illumination volume. Due to the minimal scanning requirement of the present disclosure, a handheld 3D imaging device can also be made, which could be particularly useful for clinical applications such as intraoperative surgery. Moreover, the high imaging speed of the present disclosure is critical for clinical practice in order to reduce motion artifacts, patient discomfort, cost, and risks associated with minimally invasive procedures such as endoscopy.

The embodiments described in detail herein employ a tunable dye laser pumped by an Nd:YLF laser as the irradiation source. The laser pulse duration is on the order of several nanoseconds. The pulse repetition rate, which is controlled by an external triggering signal, can be as high as a few kilohertz (kHz), without significant degradation of the output energy. In other embodiments, a plurality of sources of penetrating radiation, which can be confined to or concentrated in a small volume within the object, can be used. Such sources include, but are not limited to, pulsed lasers, flash lamps, other pulsed electromagnetic sources, particle beams, or their intensity-modulated continuous-wave counterparts.

To provide multi-focus optical illumination for photoacoustic excitation, the embodiments described in detail herein use either a microlens array or a transmission grating in conjunction with an objective lens; for photoacoustic signal detection, an ultrasonic array is used. However, the present disclosure includes any realization of light focusing using any kind of mirrors, lenses, fibers, and/or diaphragms that can produce multi-focus illumination confined to the field of view of a linear or matrix ultrasonic array. In scattering biological tissue, the system can image ˜1 mm deep, with axial and lateral resolutions determined by the ultrasonic bandwidth and the optical focusing, respectively. With an oscillating mirror, the present disclosure provides rapid optical scanning for 3D imaging, enabling optical-resolution photoacoustic microscopy at high speed.

The imaging procedure described herein is one of the possible embodiments specifically aimed at medical and biological applications. The optical absorption contrast of the present disclosure is complementary to other contrasts that can be obtained from purely optical or ultrasonic imaging technologies, and can be used for diagnostic, monitoring, or research purposes. The main applications of the technology include, but are not limited to, the imaging of arteries, veins, and pigmented tumors (such as melanomas) in vivo in humans or animals. The present disclosure can use the spectral properties of intrinsic optical contrast to monitor blood oxygenation, blood volume (total hemoglobin concentration), and even the metabolic rate of oxygen; it can also use the spectral properties of a variety of dyes or other contrast agents to obtain additional functional or molecular information. In short, the present disclosure is capable of functional and molecular imaging. In addition, the present disclosure can be used to monitor possible tissue changes during x-ray radiation therapy, chemotherapy, or other treatment; it can also be used to monitor topical application of cosmetics, skin creams, sun-blocks, or other skin treatment products.

To translate photoacoustic imaging into clinical practice, a high imaging speed is needed to reduce motion artifacts, cost, patient discomfort, and most important, the risks associated with minimally invasive procedures (e.g., endoscopy). Embodiments described herein provide the combined use of multi-focus optical illumination and ultrasonic array detection can help photoacoustic imaging meet the challenges of clinical translation. In addition, embodiments of the present disclosure uses tightly focused laser illumination to provide optical diffraction limited lateral resolution, which is difficult to achieve using ultrasonic approaches. Therefore, embodiments of the present disclosure offer a method, apparatus, and system of photoacoustic imaging with high imaging speed and spatial resolution sufficient for many clinical and preclinical applications.

FIG. 1 is a schematic of the photoacoustic probe of an imaging system 100 in accordance with one embodiment of the present disclosure, where a beam-divider, such as a 2D transmission grating, and an optically transparent acoustic reflector are employed. The light from a wavelength tunable laser is focused by a condenser lens 101 onto a pinhole 102 for spatial filtering. While a photo-detector 104 is used to monitor the laser pulse energy through a sampling beam splitter 103, an eyepiece 114 is used to optically image the object's surface for alignment. To provide multi-focus matrix illumination for photoacoustic excitation, the laser beam from the pinhole is collimated by a collimating lens 105, split by a 2D transmission grating 107, and then tightly focused by a focusing device, such as an objective lens 108, into an imaging object 111, after passing through an optically transparent acoustic reflector 110. Through an acoustic coupling medium 109 (e.g., ultrasound coupling gel), the photoacoustic signal emitted by the object is reflected by the acoustic reflector and detected by a matrix ultrasonic transducer array 112, 113. Inset A of FIG. 1 shows one possible relative positioning for the focused optical spots and the matrix ultrasonic array elements. This design is suitable for integration with commercially available ultrasonic transducer arrays (1D or 2D).

FIG. 2 is a block diagram showing the overarching architecture of the present disclosure. Components include a high-repetition-rate tunable pulsed laser system, an optical assembly, an ultrasonic array, a high-speed multi-channel data acquisition (DAQ) subsystem, a scanning mirror or linear scanner, and a multi-core computer. The optical assembly receives pulsed laser light and provides multi-focus illumination for photoacoustic excitation. The DAQ system records and digitizes the received photoacoustic signal. The laser pulse generation, data acquisition, and scanning of the optical illumination are synchronized using triggering signals from the DAQ card. To optimize the data acquisition and imaging speed, the number of data acquisition channels should match the number of elements of the ultrasonic array. However, when the number of array elements is greater, multiplexers may be used. An off-the-shelf multi-core computer, together with a parallel computing program based on Microsoft Visual Studio or other software development tools, is used to perform photoacoustic reconstruction for high-speed imaging and display.

To integrate the optical focusing and the ultrasonic detection for the present disclosure, one or more of the following devices or designs can be used: (1) an optically transparent acoustic reflector; (2) an acoustically transparent optical reflector; and/or (3) direct integration in transmission mode. Examples of the integrated focusing assembly are described with reference to FIGS. 3, 4, 5, 6, 7, 8, and 9, wherein the integrated focusing assembly includes optical focusing components, a matrix ultrasonic array, and an optical-acoustic beam combiner between them.

FIG. 3 shows the integrated focusing assembly of an imaging system in accordance with another embodiment of the present disclosure, where a 2D transmission grating and a custom-built optical-acoustic beam combiner are employed. In this embodiment, a scanning device, such as a scanning mirror 301, is used for rapid optical scanning; the 2D transmission grating 302 is used to split the laser beams; and the optical-acoustic beam combiner 304, 305, 306 is used to merge the optical illumination and the ultrasonic detection coaxially. The optical-acoustic beam combiner mainly consists of an isosceles triangular prism 304 and a rhomboidal prism 306 (the two prisms are adjoined along the diagonal surfaces with a gap of approximately 0.1 mm in between). The gap is filled with an optical refractive-index-matching, low-acoustic-impedance, nonvolatile liquid 305 (e.g., 1000 cSt silicone oil). The silicone oil and the glass have a good optical refractive index match (glass: 1.5; silicone oil: 1.4) but a large acoustic impedance mismatch (glass: 12.1×10⁶ N·s/m³; silicone oil: 0.95×10⁶ N·s/m³). As a result, the silicone oil layer is optically transparent but acoustically reflective. The laser beams coming from the grating are tightly focused by an objective lens 303 into an imaging object 308. Through an acoustic coupling medium 307, the photoacoustic signal emitted from the object is detected by a matrix ultrasonic array 309, 310. Note that within the bandwidth of the ultrasonic array, ultrasonic absorption in the silicone oil is high enough to dampen acoustic reverberations in the matching layer and thus minimize interference with the image.

FIG. 4 shows the integrated focusing assembly of an imaging system in accordance with yet another embodiment of the present disclosure, where a 2D transmission grating and an acoustically transparent thin-film optical reflector are employed. In this embodiment, a scanning mirror 401 is used for rapid optical scanning; the 2D transmission grating 402 is used to split the laser beams; and the acoustically transparent optical reflector 405 (e.g., an aluminized Mylar thin film) is used to merge the optical illumination and the ultrasonic detection coaxially. The laser beams coming from the grating are tightly focused by an objective lens 403 into an imaging object 406. Through an acoustic coupling medium 404, the photoacoustic signal emitted from the object is detected by a matrix ultrasonic array 407, 408.

FIG. 5 shows the integrated focusing assembly of an imaging system in accordance with yet another embodiment of the present disclosure, where a 2D transmission grating and a transmission-mode design are employed. In this embodiment, a scanning mirror 501 is used for rapid optical scanning; the 2D transmission grating 502 is used to split the laser beams; and the transmission-mode design is used to merge the optical illumination and the ultrasonic detection coaxially. The laser beams coming from the grating are tightly focused by an objective lens 503 into an imaging object 504. From the other side of the object, through a coupling medium 505, the photoacoustic signal emitted from the object is detected by a matrix ultrasonic array 506, 507.

FIG. 6 shows the photoacoustic probe of an imaging system in accordance with yet another embodiment of the present disclosure, where a 2D microlens array and an optically transparent acoustic reflector are employed. The light from a wavelength tunable laser is focused by a condenser lens 601 onto a pinhole 602 for spatial filtering. A photo-detector 604 is used to monitor the laser pulse energy through a sampling beam splitter 603, and an eyepiece 612 is used to optically image the object's surface for alignment. To provide multi-focus matrix illumination for photoacoustic excitation, the laser beam from the pinhole is first collimated by a collimating lens 605, and then split and focused by the 2D microlens array 606 into an imaging object 609, after passing through the optically transparent acoustic reflector 608. Through an acoustic coupling medium 607, the photoacoustic signal emitted from the object is reflected by the acoustic reflector and detected by a matrix ultrasonic array 610, 611. Inset B of FIG. 6 shows one possible relative positioning for the 2D microlens array and the matrix ultrasonic array. To produce a 3D image, raster scanning of the optical illumination is required, which, however, can be very fast, due to the small scanning area.

FIG. 7 shows the integrated focusing assembly of an imaging system in accordance with yet another embodiment of the present disclosure, where a 2D microlens array and a custom-built optical-acoustic beam combiner are employed. To provide multi-focus matrix illumination for photoacoustic excitation, the laser beam from a pinhole 701 is first collimated by a collimating lens 702, and then split and focused by the 2D microlens array 703 into an imaging object 708, after passing through the optical-acoustic beam combiner 704, 705, 706. The optical-acoustic beam combiner mainly consists of an isosceles triangular prism 704 and a rhomboidal prism 706 (the two prisms are adjoined along the diagonal surfaces with a gap of approximately 0.1 mm in between). The gap is filled with an optical refractive-index-matching, low-acoustic-impedance, nonvolatile liquid 705 (e.g., 1000 cSt silicone oil). The silicone oil and the glass have a good optical refractive index match (glass: 1.5; silicone oil: 1.4) but a large acoustic impedance mismatch (glass: 12.1×10⁶ N·s/m³; silicone oil: 0.95×10⁶ N·s/m³). As a result, the silicone oil layer is optically transparent but acoustically reflective. Through an acoustic coupling medium 707, the photoacoustic signal emitted from the object is detected by a matrix ultrasonic array 709, 710. To produce a 3D image, raster scanning of the optical illumination is required, which, however, can be very fast, due to the small scanning area.

FIG. 8 shows the integrated focusing assembly of an imaging system in accordance with yet another embodiment of the present disclosure, where a 2D microlens array and an acoustically transparent thin-film optical reflector are employed. To provide multi-focus matrix illumination for photoacoustic excitation, the laser beam from a pinhole 801 is collimated by a collimating lens 802, reflected by a dielectric mirror 803, and then split and focused by the 2D microlens array 804 into an imaging object 807. The acoustically transparent optical reflector 806 (e.g., an aluminized Mylar thin film) is used to merge the optical illumination and the ultrasonic detection coaxially. Through an acoustic coupling medium 805, the photoacoustic signal emitted from the object is detected by a matrix ultrasonic array 808, 809. To produce a 3D image, raster scanning of the optical illumination is required, which, however, can be very fast, due to the small scanning area.

FIG. 9 shows the integrated focusing assembly of an imaging system in accordance with yet another embodiment of the present disclosure, where a 2D microlens array and a transmission-mode design are employed. The transmission-mode design is used to merge the optical illumination and the ultrasonic detection coaxially. To provide multi-focus matrix illumination for photoacoustic excitation, the laser beam from a pinhole 901 is first collimated by a collimating lens 902, and then split and focused by the 2D microlens array 903 into an imaging object 904. From the other side of the object, through a coupling medium 905, the photoacoustic signal emitted from the object is detected by a matrix ultrasonic array 906, 907. To produce a 3D image, raster scanning of the optical illumination is required, which, however, can be very fast, due to the small scanning area.

The embodiments described above are based on multi-focus matrix illumination in conjunction with matrix ultrasonic array detection. However, as mentioned above, the design can be simplified to 1D, using multi-focus 1D array illumination in conjunction with linear ultrasonic array detection. With optical scanning, this simplified design can provide real-time photoacoustic B-scan imaging, sufficient for many biomedical applications. By adding one additional mechanical scanning, 3D images can be produced at a significantly higher speed compared with mechanical scanning single-focus optical-resolution photoacoustic microscopy. In addition, the implementation of this simplified version of the present disclosure can be relatively easy and inexpensive.

The above-described embodiments have been successfully demonstrated for biomedical applications. FIG. 8 illustrates a system prototype based on the transmission mode design, and using multi-focus 1D array illumination in conjunction with linear ultrasonic array detection. The system employs a tunable dye laser pumped by an Nd:YLF laser as the irradiation source. The laser pulse duration is approximately 7 nanoseconds (ns), and the pulse repetition rate can be as high as 1.5 kHz without significant degradation of the output energy. For photoacoustic excitation, a 1D microlens array consisting of 20 micro-lenses with a center-to-center spacing of approximately 250 micrometers is used, which provides twenty focused illumination spots simultaneously. For photoacoustic signal detection, a 30-MHz linear ultrasonic array consisting of forty-eight elements with a spacing of approximately 100 micrometers is used. The system can image approximately 1 mm deep in scattering biological tissue, with finer than 10-micrometer lateral resolution and an approximately 25-micrometer axial resolution. Currently, the system acquires a volume image data set of 1000 by 500 by 200 voxels in approximately 4 min, which is about 4 times faster than existing mechanical scanning single-focus photoacoustic microscopy with optical resolution. In principle, using a 48-channel DAQ system to eliminate the 6:1 multiplexing, the system can image about 20 times faster, as determined primarily by the number of optical foci. With an increased laser repetition rate and more densely packed micro-lens array, the imaging speed of the system can be further improved. Of note, the 10-micrometer optically defined lateral resolution is one to two orders of magnitude finer than the acoustically defined counterpart, enabling the system to image capillary-level microvessels in vivo.

FIG. 10A shows a photoacoustic maximum amplitude projection image of two crossed 6-micrometer diameter carbon fibers acquired at 570 nm. Maximum amplitude projection refers to projection of the maximum photoacoustic amplitudes along a direction—usually the depth or z axis direction unless otherwise mentioned—to its orthogonal plane. FIG. 10B shows the distribution of the photoacoustic amplitude across the imaged vertical fiber (along the dashed line in FIG. 10A), demonstrating that the lateral resolution of the system is at least as fine as 10 micrometers.

FIGS. 11A and 11B show a photoacoustic maximum amplitude projection and a 3D image of a mouse ear microvasculature, respectively, acquired noninvasively in vivo. Microvessels, including those in the capillary-level (less than approximately 10 micrometers), are clearly imaged. With high imaging speed and optically defined spatial resolution, the preliminary results demonstrate the potential of the present disclosure for broad biomedical applications. For example, imaging speed is one critical issue in advancing photoacoustic endoscopy into clinical practice for early cancer detection or intravascular atherosclerosis imaging. In addition, the high-speed, high-resolution capability will open up new possibilities for the study of tumor angiogenesis, diabetes-induced vascular complications, and pharmacokinetics.

FIG. 12 is a flowchart 1200 that illustrates an exemplary imaging method. In some embodiments, a scanning device, such as scanning mirror 301, a galvanometer, a polygon scanner, an acoustic-optical scanner, and an electro-optical scanner, or another suitable scanning device, receives 1202 a first light beam from a light source, such as a laser or laser system. A beam-divider, such as transmission grating 107 or 2D microlens array 606, splits 1204 the first light beam into a plurality of second light beams. A focusing device, such as objective lens 108 or collimating lens 605, focuses 1206 the second light beams on respective locations in object of interest 111. In response to the second light beams, the temperature within object 111 at each location rises, which leads to a transient thermal expansion, resulting in emission of photoacoustic signals by object 111. The photoacoustic signals are received 1208 by ultrasonic transducer array 112, for example.

In some embodiments, a computer generates an image based on the photoacoustic signals and displays the image to an operator. Moreover, in some embodiments, the second light beams and the photoacoustic signals are coaxially merged by optical-acoustic beam combiner 304, 305, 306 that includes a first prism, such as isosceles triangular prism 304, and a second prism, such as rhomboidal prism 306, separated from first prism 304 by a gap filled with non-volatile liquid 305.

Embodiments described herein provide methods, systems, and apparatus for high-speed, optical-resolution photoacoustic imaging using multi-focus optical illumination in conjunction with ultrasonic array detection. For example, embodiments of the present disclosure use multiple focused pulsed laser beams to produce a rapid local temperature rise at multiple optical foci from absorption of the pulsed light. The temperature rise leads to a transient thermal expansion, resulting in photoacoustic emissions, which are detected by a high-frequency ultrasonic array to reconstruct an image. The image signal amplitude is related to the optical absorption and the Grueneisen parameter. With each laser pulse, the multi-focus illumination excites photoacoustic waves from multiple sites, which the ultrasonic array detects simultaneously. With an appropriate reconstruction algorithm, the signals from the multiple sites are separated. Using multi-focus matrix illumination and matrix ultrasonic array detection, a three-dimensional (3D) photoacoustic image can be produced by rapidly scanning the illumination over a small area comparable in size with the lateral resolution of the ultrasonic array. Consequently, a relatively large 3D volume (e.g., 10 mm×10 mm×1 mm) can be imaged at high speed—even in real time—with optical diffraction limited lateral resolution (e.g., 5 micrometers). In one embodiment of the present disclosure, multi-focus one-dimensional (1D) array illumination and linear ultrasonic array detection can be used. Even with this simplified design, the multi-focus optical-resolution photoacoustic microscopy device demonstrates a significant improvement in imaging speed over previous optical-resolution photoacoustic microscopy devices. Overall, embodiments of the present disclosure provide 3D photoacoustic microscopy of optical absorption contrast with optical diffraction limited lateral resolution at high speed.

It will be understood that particular embodiments described herein are shown by way of illustration and not as limitations of the disclosure. The principal features of this disclosure can be employed in various embodiments without departing from the scope of the disclosure. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, numerous equivalents to the specific procedures described herein. Such equivalents are considered to be within the scope of this disclosure and are covered by the claims.

All of the compositions and/or methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this disclosure have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations can be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the disclosure. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the disclosure as defined by the appended claims

It will be understood by those of skill in the art that information and signals may be represented using any of a variety of different technologies and techniques (e.g., data, instructions, commands, information, signals, bits, symbols, and chips may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof). Likewise, the various illustrative logical blocks, modules, circuits, and algorithm steps described herein may be implemented as electronic hardware, computer software, or combinations of both, depending on the application and functionality. Moreover, the various logical blocks, modules, and circuits described herein may be implemented or performed with a general purpose processor (e.g., microprocessor, conventional processor, controller, microcontroller, state machine or combination of computing devices), a digital signal processor (“DSP”), an application specific integrated circuit (“ASIC”), a field programmable gate array (“FPGA”) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. Similarly, steps of a method or process described herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. Although preferred embodiments of the present disclosure have been described in detail, it will be understood by those skilled in the art that various modifications can be made therein without departing from the spirit and scope of the disclosure as set forth in the appended claims.

A controller, computer, or computing device, such as those described herein, includes at least one processor or processing unit and a system memory. The controller typically has at least some form of computer readable media. By way of example and not limitation, computer readable media include computer storage media and communication media. Computer storage media include volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules, or other data. Communication media typically embody computer readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism and include any information delivery media. Those skilled in the art are familiar with the modulated data signal, which has one or more of its characteristics set or changed in such a manner as to encode information in the signal. Combinations of any of the above are also included within the scope of computer readable media.

Although the present disclosure is described in connection with an exemplary imaging system environment, embodiments of the disclosure are operational with numerous other general purpose or special purpose imaging system environments or configurations. The imaging system environment is not intended to suggest any limitation as to the scope of use or functionality of any aspect of the disclosure. Moreover, the imaging system environment should not be interpreted as having any dependency or requirement relating to any one or combination of components illustrated in the exemplary operating environment.

Embodiments of the disclosure may be described in the general context of computer-executable instructions, such as program components or modules, executed by one or more computers or other devices. Aspects of the disclosure may be implemented with any number and organization of components or modules. For example, aspects of the disclosure are not limited to the specific computer-executable instructions or the specific components or modules illustrated in the figures and described herein. Alternative embodiments of the disclosure may include different computer-executable instructions or components having more or less functionality than illustrated and described herein.

When introducing elements of aspects of the disclosure or embodiments thereof, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

This written description uses examples to disclose the disclosure, including the best mode, and also to enable any person skilled in the art to practice the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. 

What is claimed is:
 1. An imaging method comprising: receiving a first light beam from a light source; splitting the first light beam into a plurality of second light beams using a beam-divider; focusing the plurality of second light beams on respective locations in an object of interest using a focusing device, the plurality of second light beams causing the object of interest to emit acoustic signals; and receiving the acoustic signals from the object of interest using an ultrasonic transducer array.
 2. An imaging method in accordance with claim 1, wherein the plurality of second light beams and acoustic signals are coaxially aligned on opposite sides of the object of interest in a transmission mode.
 3. An imaging method in accordance with claim 1, further comprising coaxially merging the plurality of second light beams and the acoustic signals using an optical-acoustic beam combiner.
 4. An imaging method in accordance with claim 3, wherein coaxially merging the plurality of second light beams and the acoustic signals comprises passing the plurality of second light beams through the optical-acoustic beam combiner to the focusing device and reflecting the acoustic signals entering the optical-acoustic beam combiner toward the ultrasonic transducer array.
 5. An imaging method in accordance with claim 3, wherein coaxially merging the plurality of second light beams and the acoustic signals comprises reflecting the plurality of second light beams entering the optical-acoustic beam combiner toward the focusing device and passing the acoustic signals through the optical-acoustic beam combiner to the ultrasonic transducer array.
 6. An imaging method in accordance with claim 1, further comprising reflecting the first light beam toward the beam-divider using a movable scanning mirror.
 7. An imaging method in accordance with claim 6, wherein the first light beam is reflected in a raster scanning pattern.
 8. An imaging method in accordance with claim 1, further comprising generating an image based on the acoustic signals.
 9. An imaging method in accordance with claim 1, wherein the plurality of second light beams are formed as a linear array.
 10. An imaging method in accordance with claim 9, wherein the acoustic signals are received by a linear ultrasonic transducer array.
 11. An imaging method in accordance with claim 1, wherein the plurality of second light beams are formed as a 2D array.
 12. An imaging method in accordance with claim 11, wherein the acoustic signals are received by a 2D ultrasonic transducer array. 